Transport And Translocation Of Water And Solutes PDF

TRANSPORT AND TRANSLOCATION OF WATER AND SOLUTES Vascular tissue  Transports nutrients throughout a plant; such transport may occur over long distances Figure 36.1 TRANSPORT IN PLANTS Three levels: 1. at cellular level (root hairs) 2. lateral tran...

TRANSPORT AND TRANSLOCATION OF WATER AND SOLUTES Vascular tissue  Transports nutrients throughout a plant; such transport may occur over long distances Figure 36.1 TRANSPORT IN PLANTS Three levels: 1. at cellular level (root hairs) 2. lateral transport (short-distance) 3. whole plant (long distance) TRANSPORT IN PLANTS Cellular Level:  Diffusion  Osmosis  Active Transport TRANSPORT IN PLANTS Cellular Level:  Water potential  Addition of solutes lowers  Increasing pressure raises  Water moves from high water potential to low water potential  Turgor pressure  Water and minerals ascend from roots to shoots through the xylem  Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant  The transpired water must be replaced by water transported up from the roots SOIL-PLANT-ATMOSPHERE CONTINUUM 2.4.3 Water moves from soil to plant to atmosphere  WATER POTENTIAL  Plants need to acquire water, move it through their structures  Also lose water to the environment  All these depend on water potential of various plant parts, immediate environment  WATER POTENTIAL  Water potential - difference in potential energy between pure water and water in some system  Represents sum of osmotic, pressure, matric, and gravitational potentials  WATER POTENTIAL  Water always moves from larger to smaller water potentials  Pure water has water potential of 0  Soils, plant parts have negative water potentials  Gradient in water potential drives water from soil, through plant, into atmosphere The cohesion-tension theory explains the movement of water from the roots to a leaf of a plant. 1. Through Xylem 2. No metabolic energy required 3. Depends on physical-chemical properties of water, driven by water potential. Stick to each other and adhere to cell wall.  WATER POTENTIAL  Energy is required to move water upward through plant into atmosphere  Energy not expended by plant itself  Soil to roots - osmotic potential  Up through tree and out - pressure potential  Sunlight provides energy to convert liquid into vapor TRANSPIRATION - WATER LOSS  Transpiration caused by huge difference in water potential between moist soil and air  Huge surface area of roots, leaves produce much higher losses via transpiration than evaporative losses from open body of water TRANSPORT IN PLANTS Lateral Transport How water & dissolved minerals get into roots...  through the cells of the root  along the extracellular pathway consisting of cell walls  Lateral transport of minerals and water in roots Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast 1 Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls. Casparian strip Plasma 2 Minerals and water that cross membrane the plasma membranes of root Apoplastic 1 hairs enter the symplast. route Vessels 2 3 As soil solution moves along (xylem) the apoplast, some water and Root minerals are transported into Symplastic the protoplasts of cells of the route hair epidermis and cortex and then move inward via the symplast. Epidermis Cortex Endodermis Vascular cylinder 4 Within the transverse and radial walls of each endodermal cell is the 5 Endodermal cells and also parenchyma cells within the Casparian strip, a belt of waxy material (purple band) that blocks the vascular cylinder discharge water and minerals into their passage of water and dissolved minerals. Only minerals already in walls (apoplast). The xylem vessels transport the water the symplast or entering that pathway by crossing the plasma and minerals upward into the shoot system. membrane of an endodermal cell can detour around the Casparian Figure 36.9 strip and pass into the vascular cylinder. FIGURE 36.8 MYCORRHIZAE, SYMBIOTIC ASSOCIATIONS OF FUNGI AND ROOTS LATERAL TRANSPORT OF MINERALS AND WATER IN ROOTS CASPARIAN STRIP  A waxy material that surrounds endodermal cells  prevents material from crossing the endodermis between cells  Substances must enter the cells of the endodermis in order to pass into the vascular cylinder  Allows selectivity TRANSPORT IN PLANTS Long Distance Transport  Diffusion too slow  Instead substances move by bulk flow  the movement of fluid due to pressure XYLEM  dead at functional maturity  Only the cell walls remain to form tubes, connected by pores, through which water can move. XYLEM  Xylem sap brings minerals to leaves and water to replace what is lost by transpiration  Transpiration is the evaporation of water from leaves or other aerial parts of the plant  rates of >15 m per hour  distances of 100m in the tallest trees. MOVEMENT OF XYLEM SAP Pushed or Pulled? MOVEMENT OF XYLEM SAP Pushed…  by root pressure  minerals actively pumped into the xylem  causes water to move in by osmosis  positive pressure is generated  When transpiration is low  Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous eudicots MOVEMENT OF XYLEM SAP Pulled…  by transpiration-cohesion  Water is pulled out of the xylem to replace the water that is lost from leaves through stomata  Xylem sap Outside air Y = –100.0 MPa Mesophyll cells Stoma Leaf Y (air spaces) Water = –7.0 MPa molecule Transpiration Leaf Y (cell walls) Atmosphere = –1.0 MPa Xylem cells Adhesion Cell wall Water potential gradient Trunk xylem Y = – 0.8 MPa Cohesion, by Cohesion hydrogen and adhesion bonding in the xylem Water Root xylem Y molecule = – 0.6 MPa Root Soil Y hair = – 0.3 MPa Soil particle Water uptake Water Figure 36.13 from soil MOVEMENT OF XYLEM SAP Pulled…  transpiration pull is translated all the way to the roots by  cohesion  Adhesion  Costs no energy to transport xylem sap up to leaves PHLOEM  cells that are living at functional maturity  Phloem sap consists primarily of sugar, primarily sucrose (30%)  hormones, amino acids, minerals PHLOEM SAP  Direction of flow in phloem is variable  Bulk Flow or Mass Flow Hypothesis  PHLOEM SAP  flows from sugar sources  where sugar is produced by photosynthesis or the breakdown of starch  Leaves, storage organs  to sugar sinks  that consume sugar  non green plant parts, growing shoots and roots, fruits PHLOEM SAP  sugars are actively loaded into phloem at the source  water follows and there is a high pressure  sugars are actively transported out of the phloem at the sink  water follows and pressure is lower  sap flows from high to low pressure